I. Field of the Invention
This invention relates to gasification of sulfur and hydrocarbon containing streams in molten metal.
II. Description of the Prior Art
Molten metal, especially molten iron, baths are well known and widely used as gasifiers. The light temperatures in such baths rapidly decompose, by thermal action, a variety of solid, liquid and gaseous feeds into hydrogen and/or carbon oxides. Such processes are well known, e.g., U.S. Pat. Nos. 4,574,714 and 4,602,574 to Bach teach a molten iron gasifier. Another, and preferred, molten metal reactor is disclosed in U.S. Pat. No. 5,435,814, MOLTEN METAL DECOMPOSITION APPARATUS, Charles B. Miller and Donald P. Malone.
One of the problems of molten metal processing is that the feeds to such processes are rarely pure materials. If the feed were a pure hydrocarbon, such as methane with no significant amount of chlorides, trash metals, sulfur or other impurities, design and operation of the molten metal reactor is simple. Consider the case of methane conversion. The reactor need only be designed to thermally convert the CH.sub.4 into hydrogen (which is thermally stable and rapidly released as pure hydrogen gas) and carbon (which rapidly dissolves in the molten iron). There are no feed impurities and no slag forms.
Once the refiner has to depart from such ideal fuels as methane, design of the reactor becomes complicated. Molten metal reactors are best at converting difficult streams not otherwise amenable to processing--resids, ground up tires, old pesticides and the like. Such materials present many challenges to the engineer charged with converting them to useful products (or at least making the offending material go away), but for now the focus is on one pervasive impurity--sulfur. The problem of sulfur in the feed is pervasive in refinery processing, coal combustion and molten metal processing. It is instructive to review how each of these processes has dealt with feed sulfur.
Crude oil invariably contains sulfur. Sulfur is so pervasive in crude oil that its presence in greater or lessor amounts makes crude oil sour or sweet. Refiners have evolved efficient ways to convert sulfur in feed into solid sulfur product. Sulfur is a valuable product in its elemental form. In refineries, the crude is generally catalytically hydrotreated to convert sulfur compounds to H.sub.2 S which is eventually converted in a Claus unit to elemental sulfur. The processing is expensive, both in terms of operating and capital expense required to hydrotreat feeds, but essential.
In coal processing, sulfur is generally dealt with by stack gas scrubbing or by burning the coal in a bed of ground up limestone or dolomite. It is possible to burn coal in California if a Circulating Fluidized Bed (CFB) coal combustor is used. Relatively small amounts of coal are added to a much larger circulating inventory of crushed alkaline material. The sulfur components in the coal are oxidized to form sulfur oxides, which then react with tons of circulating, high temperature, ground dolomite.
In molten metal processing (and to some extent in steel making), sulfur is oxidized during processing to form sulfur oxides. Tie produced sulfur oxides then react with alkaline material added to the bath to form a slag layer. One example of this approach is a vitreous layer used above a molten metal bath, as taught in U.S. Pat. No. 5,354,940. A typical vitreous layer was five inches of 40% calcium oxide, 40% silicone dioxide and 20% aluminum oxide.
Although it has been known for years that it is possible to release some H.sub.2 S from a molten iron bath no one has made any productive use of this finding. In UK 1,187,782, Nixon taught that a molten metal conversion process converting methane to hydrogen would also refine the iron bath:
The hydrogen produced in the cracking zone has a refining effect on the metal contained in the molten metal bath in contact with it. For instance, the sulphur content of the molten iron tends to be reduced as a result of the reaction: EQU FeS+H2=Fe+H.sub.2 S.
The EXAMPLE in the Nixon patent showed conversion of methane to high purity hydrogen (H2 volume % purity was 99.86 and 99.70). The sulfur content of the hydrogen gas was not reported, but presumably trace amounts of H.sub.2 S were present, at least during the early stages of the process as sulfur present in tile iron was removed by the refining effect of the hydrogen production. In this example, the sulfur was in the metal and no sulfur in the feed.
Some use has been made of molten metal baths to absorb H.sub.2 S.sub.x though the bath in question was not a molten iron bath and operated at a lower temperature than molten iron baths.
I did not like the conventional approach to dealing with feed sulphur. Now, refiners add large amounts of alkaline material to form an alkaline slag layer which has to be thick enough to react with sulphur compounds as they form or extract dissolved sulfur from the metal. The added alkaline material consumes significant amounts of energy when dumped into the reactor to form a slag layer. This slag layer in turn creates a removal problem and eventually a disposal problem.
I wanted to be able to deal with sulfur containing feeds without unnaturally altering the heat balance of the reactor by adding large amounts of alkaline material to deal with the feed sulfur. I discovered a way convert much, and potentially all, of the feed sulfur to H.sub.2 S. Such material, while highly toxic, is easily handled in any modern refinery using conventional amine scrubbing, Claus conversion and the like techniques. H.sub.2 S is a dangerous material, but refiners have been efficiently converting it to elemental sulfur for over 50 years.
The solution was surprisingly simple. Change the operating conditions in the molten metal reactor so that strong reducing environment was created. Rather than do this adding hydrogen at ruinous expense, do it cheaply by letting the carbon level build up in the reactor. High dissolved carbon levels in a molten iron bath could be used to create conditions where most, or all, of the feed sulfur could be converted to H.sub.2 S.